WO2017028890A1

WO2017028890A1 - Method of controlling operation of an interface arrangement in a power transmission system

Info

Publication number: WO2017028890A1
Application number: PCT/EP2015/068830
Authority: WO
Inventors: Praveen Kumar Barupati; Sasitharan Subramanian; Sivaprasad JALDANKI; Subhasish Mukherjee
Original assignee: Abb Schweiz Ag
Priority date: 2015-08-17
Filing date: 2015-08-17
Publication date: 2017-02-23

Abstract

A control and processing module (170) and a method of controlling operation of a thyristor-based interface arrangement (100) configured to electrically couple an Alternating Current (AC) power system (110) with a Direct Current (DC) power system (120), and to convert DC power to AC power, or vice versa, are disclosed. The interface arrangement (100) comprises a plurality of converter valves (131-136, 141-143), each comprising at least one thyristor (T_1x, T_1y; T_2x, T_2y;...; T_9x, T_9y), and a plurality of multi-level converter cells (WS1, WS2). By means of controlling the firing angle of the at least one thyristor (T_1x, T_1y; T_2x, T_2y;...; T_9x, T_9y) of each converter valve (131-136, 141-143), shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells (WS1, WS2) may be adjusted, and so that reactive power exchange between the interface arrangement (100) and the AC power system (110) may be controlled. A system comprising the interface arrangement (100) and the control and processing module (170) is also disclosed.

Description

METHOD OF CONTROLLING OPERATION OF AN INTERFACE ARRANGEMENT IN

A POWER TRANSMISSION SYSTEM

TECHNICAL FIELD

The present invention generally relates to the field of power transmission systems, e.g. High Voltage Direct Current (HVDC) power transmission systems. Specifically, the present invention relates to an interface arrangement between an alternating current (AC) power system and a direct current (DC) power system, e.g. a converter station between an AC power system and a DC power system, and a control and processing module and a method for controlling operation of the interface arrangement.

BACKGROUND

Interface arrangements are known to be connected between an AC power system and a DC power system. Such an arrangement typically includes a converter, such as a voltage source converter, for conversion of AC power to DC power, or vice versa. The interface arrangement has a DC side for coupling to the DC power system and an AC side for coupling to the AC power system. The arrangement often includes a transformer having a primary side connected to the AC system and a secondary side for coupling to the converter.

HVDC power transmission has become increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems. For example in a HVDC power system, there is generally included an interface arrangement including or constituting an HVDC converter station, which is a type of station configured to convert high voltage DC to AC, or vice versa. An HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), one or more transformers, capacitors, filters, and/or other auxiliary elements. Converters may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized as line commutated converters (LCCs) or voltage source converters (VSCs), e.g. depending on the type of switches (or switching devices) which are employed in the converter. A plurality of solid-state

semiconductor devices such as IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter.

HVDC Classic is a technology developed by ABB and that is based on LCC technology. HVDC Light is a technology developed by ABB and that is based on VSC technology. While HVDC Classic converters employ thyristors as switches or switching elements (and/or other switches or switching elements that are not self-commutated), HVDC Light converters employ IGBTs as switches or switching elements (and/or other switches or switching elements that are self-commutated). While HVDC Classic converters may be used for relatively high power ranges, they may exhibit lower order harmonics in the AC current and may require reactive power from the AC grid or power system. HVDC Light converters may be capable of independently controlling the active and reactive power handled by the converter, and may provide a relatively smooth voltage waveform, possibly a substantially sinusoidal or sinusoidal one, because of the multilevel nature of these converters. But the power ratings that may be achieved with HVDC Light converters may be less than that which may be achieved with HVDC Classic converters. A thyristor-based VSC employs a combination of switches or switching elements in the form of thyristors and self-commutated switches or switching elements such as IGBTs in HVDC applications. However, it may be difficult or even impossible to control the reactive and active power independently using such a converter.

SUMMARY

In view of the above, a concern of the present invention is to provide means for facilitating or even enabling controlling reactive power transfer between an interface arrangement and an AC power system, which interface arrangement is configured to electrically couple the AC power system and a DC power system,.

To address at least one of this concern and other concerns, a control and processing module configured to control operation of an interface arrangement, a system and a method of controlling operation of an interface arrangement in accordance with the independent claims are provided. Preferred embodiments are defined by the dependent claims.

According to a first aspect, there is provided a control and processing module which is configured to control operation of an interface arrangement. The interface

arrangement is configured to electrically couple an AC power system with a DC power system and to convert DC power to AC power, or vice versa. The interface arrangement is configured to provide at least a portion of an AC waveform. The interface arrangement comprises a plurality of multi-level converter cells, each of which is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system. The interface arrangement comprises a plurality of electrically connected converter modules. Each of the converter modules comprises at least one converter valve electrically connected to the plurality of multi-level converter cells and comprising at least one thyristor having a controllable firing angle. Shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells are adjustable by controlling firing angle of the at least one thyristor of each converter valve. The control and processing module according to the first aspect is configured to:

based on a desired reactive power exchange between the interface arrangement and the AC power system, determine a corresponding AC voltage at the AC side of the interface arrangement;

based on the voltage of the DC power system and the determined AC voltage, determine a firing angle of the at least one thyristor of each converter valve that will result in a or any voltage contribution to the AC waveform provided by the plurality of multilevel converter cells conforming to the determined AC voltage; and

- control the firing angle of the at least one thyristor of each converter valve in accordance with the firing angle as determined for the respective thyristor.

Thus, by controlling the firing angle of the at least one thyristor of each converter valve in accordance with the firing angle as determined for the respective thyristor, an AC voltage at the AC side of the interface arrangement may be achieved that will correspond to the desired reactive power exchange between the interface arrangement and the AC power system. In that manner, reactive power exchange between the interface

arrangement and the AC power system may be controlled. In turn, this may reduce or even eliminate need for any capacitor filters for reactive power compensation on the AC side of the interface arrangement, which may allow for or facilitate a reduction in both size and cost of the overall power transmission system.

In the context of the present application, by a voltage contribution to the AC waveform conforming to an AC voltage (e.g., the determined AC voltage) it is meant that the voltage contribution to the AC waveform equals or substantially equals the AC voltage (but some variation may be possible, e.g. by a few percent).

In the context of the present application, by a firing angle of a thyristor it is meant a phase angle of an applied voltage waveform at which the thyristor is 'fired', or starts to conduct current, i.e. is triggered into electrical conduction. Another way to describe the firing angle of a thyristor is an angular difference, e.g. in degrees, along a 360 degrees (or 2π radians) periodic (e.g., sinusoidal) waveform between the point of the ('natural') firing of the thyristor and the point at which the thyristor is actually triggered into conduction, i.e. the firing angle. That is to say, the firing angle of a thyristor may be considered as the phase angle between zero-crossing of the AC voltage and the firing instant of the thyristor.

The control and processing module may be configured to control the firing angle of each thyristor individually, or it may be configured to control the firing angles of all thyristors as a group.

The control and processing module may be configured to control the firing angle of the at least one thyristor of each converter valve for example within a range between about 5° to about 27°. According to a second aspect, there is provided a system comprising an interface arrangement configured to electrically couple an AC power system with a DC power system and to convert DC power to AC power, or vice versa. The interface arrangement is configured to provide at least a portion of an AC waveform. The interface arrangement comprises a plurality of multi-level converter cells, each of which is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system. The interface arrangement comprises a plurality of electrically connected converter modules. Each of the converter modules comprises at least one converter valve electrically connected to the plurality of multi-level converter cells and comprising at least one thyristor having a controllable firing angle. Shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells are adjustable by controlling firing angle of the at least one thyristor of each converter valve. The system comprises a control and processing module according to the first aspect configured to control operation of the interface arrangement.

At least one, some or even each of the plurality of converter valves may be line-commutated. In the context of the present application, by a line-commutated converter valve it is meant that the converter valve is not self-commutated. At least one, some or even each of the plurality of converter valves may for example comprise at least two anti-parallel thyristors, or at least one pair of anti-parallel thyristors.

In the context of the present application, by a self-commutated converter valve or switch or switching element, it is meant a converter valve or switch or switching element for which both turn-on and turn-off can be controlled. An example of a switch or switching element for which both turn-on and turn-off can be controlled is an insulated-gate bipolar transistor, IGBT. And further in the context of the present application, by a converter valve or switch or switching element that is not self-commutated, or line-commutated, it is meant a converter valve or switch or switching element for which only turn-on or turn-off can be controlled. For example, some thyristors may be relatively easily turned on, but a forced current commutation may be required in order to turn them off.

The interface arrangement may be configured such that each converter valve is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, thereby defining a plurality of converter states, so as to selectively control polarity of any voltage contribution to the AC waveform provided by the multi-level converter cells.

In the context of the present application, by a non-conducting state of a converter valve it is meant a state where there is no or only very little conduction of current through the converter valve.

By way of the controlling of the firing angle of the at least one thyristor of each converter valve, voltage across at least one of the multi-level converter cells may become non-zero at change from one converter state to another converter state. Thereby, commutation of the at least one thyristor of at least one converter valve for changing from the one converter state to the other converter state may be effected by means of the non-zero voltage across at least one of the multi-level converter cells, reverse-biasing the at least one thyristor of the at least one converter valve. Thereby, the interface arrangement may not require any dedicated commutation cell or element, configured to commutate the thyristors. In turn, the interface arrangement may become less complex, and the overall cost of the interface arrangement may be reduced (as compared to if employing dedicated commutation cells or elements).

In the context of the present invention, by commutation of a thyristor it is meant to switch the thyristor into a non-conducting state.

The interface arrangement may comprise a DC side for coupling of the interface arrangement to the DC power system and an AC side for coupling of the interface arrangement to the AC power system. The AC side and/or the DC side may for example include at least one terminal.

The AC power system may comprise a plurality of phases, wherein each of the converter modules may correspond to one of the phases. The correspondence between the converter modules and the phases may be one-to-one, and so there may be a separate, or particular converter module corresponding to each phase. The interface arrangement may hence be a multi-phase arrangement.

The plurality of multi-level converter cells, each of which may be electrically connected to the DC power system, may for example be electrically connected, possibly in series, between a first DC pole and a second DC pole, or between a DC pole and ground. Thus, the interface arrangement may for example be configured according to a monopole configuration, or a bipole configuration. The interface arrangement is however not limited thereto, but may for example in alternative be configured according to an asymmetrical monopole configuration.

In the context of the present application, by a multi-level converter cell it is meant a converter cell that is configured so as to be capable of providing a multiple of (two or more) voltage levels, which may be used in forming an AC voltage (waveform).

The at least one multi-level converter cell may for example comprise a half- bridge cell or a full-bridge cell.

Each multi-level converter cell may comprise at least one electrical energy storage element (e.g., a capacitor) which can be selectively charged with DC power from the DC power system and selectively discharged. Each multi-level converter cell may be configured to provide a voltage contribution to the AC voltage waveform based on a voltage of the electrical energy storage element.

A multi-level converter cell may for example comprise at least one capacitor, and/or another type of electrical energy storage element, electrically connected, e.g. in parallel, with a series connection of switching elements, e.g. including Integrated Gate- Commutated Transistor (IGBT)-diode pairs, each IGBT-diode pair comprising one or more IGBTs and a diode arranged in anti-parallel fashion with respect to the IGBT(s).

The interface arrangement may for example be included in or constitute a converter or converter station, e.g. a HVDC converter or converter station.

According to a third aspect, there is provided a power system which includes an AC power system and a DC power system. The power system according to the third aspect comprises a system according to the second aspect, wherein the interface arrangement of the system is configured to couple the AC power system with the DC power system. The power system may for example include an HVDC power system and/or a DC grid.

According to a fourth aspect, there is provided a method of controlling operation of an interface arrangement configured to electrically couple an AC power system with a DC power system and to convert DC power to AC power, or vice versa. The interface arrangement is configured to provide at least a portion of an AC waveform. The interface arrangement comprises a plurality of multi-level converter cells, each of which is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system. The interface arrangement comprises a plurality of electrically connected converter modules. Each of the converter modules comprises at least one converter valve electrically connected to the plurality of multi-level converter cells and comprising at least one thyristor having a controllable firing angle. Shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells are adjustable by controlling firing angle of the at least one thyristor of each converter valve. The method comprises, based on a desired reactive power exchange between the interface arrangement and the AC power system, determining a corresponding AC voltage at the AC side of the interface arrangement. Based on the voltage of the DC power system and the determined AC voltage it is determined a firing angle of the at least one thyristor of each converter valve that will result in a or any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells conforming to the determined AC voltage. An average value of the voltage contribution from the multi-level converter cells may be equal to, or substantially equal to, the DC voltage (e.g. the DC voltage denoted by V_s in the description in the following with reference to the drawings). The firing angle of the at least one thyristor of each converter valve is controlled in accordance with the firing angle as determined for the respective thyristor.

According to a fifth aspect, there is provided a computer program product configured to, when executed in a control and processing module according to the first aspect, perform a method according to the fourth aspect.

According to a sixth aspect, there is provided a computer-readable storage medium on which there is stored a computer program product configured to, when executed in a control and processing module according to the first aspect, perform a method according to the fourth aspect.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments. It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the description herein. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 is a schematic circuit diagram of an interface arrangement according to an embodiment of the present invention, which interface arrangement has a thyristor-based voltage source converter topology.

Figures 2 and 3 are schematic graphs of the voltages generated by the multilevel converter cells of the wave-shaper module of the interface arrangement illustrated in Figure 1 and which are 'transferred' to the AC side of the interface arrangement by way of the converter valves of the interface arrangement.

Figures 4 and 5 are schematic graphs of the voltages generated by the multilevel converter cells of the wave-shaper module of the interface arrangement illustrated in Figure 1 and which are 'transferred' to the AC side of the interface arrangement by way of the converter valves of the interface arrangement, while employing a delay in the firing angle of the thyristors of the converter valves.

Figure 6 is a schematic circuit diagram of an interface arrangement according to an embodiment of the present invention.

Figure 7 is a schematic graph of the voltages generated by the multi-level converter cells of the wave-shaper module of the interface arrangement illustrated in Figure 6 and which are 'transferred' to the AC side of the interface arrangement by way of the converter valves of the interface arrangement, while employing a delay in the firing angle of the thyristors of the converter valves.

Figure 8 is a schematic flowchart of a method according to an embodiment of the present invention.

Figure 9 is a schematic view of computer-readable means carrying computer program code according to embodiments of the present invention. All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein;

rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

Figure 1 is a schematic circuit diagram of an interface arrangement 100 according to an embodiment of the present invention. The interface arrangement 100 is configured to electrically couple an alternating current (AC) power system 110 with a direct current (DC) power system, schematically indicated at 120, and to convert AC power to DC power, or vice versa.

The interface arrangement 100 illustrated in Figure 1 has a thyristor-based voltage source converter topology. The interface arrangement 100 may be described with reference to four main parts or portions: a main thyristor bridge 130, an interconnector 140, a wave-shaper module 160 and a commutation module 150.

The interface arrangement 100 is electrically connected to the DC power system 120 by way of a DC bus. The DC bus voltage is denoted Vs, as indicated in Figure 1.

The interface arrangement 100 illustrated in Figure 1 is a three-phase arrangement, with the three phases indicated by R, Y, and B, respectively. However, it is to be understood that the number of phases illustrated in Figure 1 is according to an exemplifying embodiment of the present invention, and that variations in the number of phases are possible. As indicated in Figure 1, in the AC power system 110 the three phases have voltages V_gR, V_gy and V_gB, respectively.

As illustrated in Figure 1 , each of the main thyristor bridge 130 and the interconnector 140 may for example comprise semiconductor switching elements in the form of pairs of anti-parallel thyristors T_lx, T_ly; T_2x, T_2y, . .. , T_9x, T_9y. As illustrated in Figure 1, the thyristors T_lx and T_ly form a first thyristor pair arranged with anti-parallel thyristors T_lx and Tiy, the thyristors T_2x and T_2y form a second thyristor pair with anti-parallel thyristors T_2x and T_2y, and so on, and the thyristors T_9x and T_9y form a ninth thyristor pair with anti-parallel thyristors T_9x and T_9y. The thyristors which in Figure 1 are denoted by subscript x may conduct current when the AC current is positive. The thyristors which in Figure 1 are denoted by subscript y may conduct current when the AC current is negative. As illustrated in Figure 1, the main thyristor bridge 130 may form a six-pulse bridge wherein the switching elements are anti-parallel thyristor pairs. However,

configurations of the switching elements other than in a six -pulse bridge are possible, such as known in the art. For example, the main thyristor bridge 130 may, according to one or more other embodiments of the present invention, form a twelve-pulse bridge.

As indicated in Figure 1, the main thyristor bridge 130 may be connected to the AC power system 110 by way of a transformer 112 (e.g., a three-phase transformer). And as also indicated in Figure 1, AC currents I_R, Ιγ, and ¾, respectively, are flowing between the transformer 112 and the main thyristor bridge 130.

The interface arrangement 100 comprises a plurality of multi-level converter cells, at least some of which may be included in or constitute the wave-shaper module 160. According to the exemplifying embodiment of the present invention illustrated in Figure 1 there are two multi-level converter cells WSl, WS2, or wave-shaper cells, included in the wave-shaper module 160. It is however possible for the interface arrangement 100 or wave- shaper module 160 to have more than two wave-shaper cells, or even a single wave-shaper cell. In accordance with the exemplifying embodiment of the present invention illustrated in Figure 1, the multi-level converter cells WSl, WS2 may be electrically connected in series and may be electrically connected to the DC power system 120 for example between a first DC pole and a second DC pole, or between a DC pole and ground. Thus, the interface arrangement 100 may for example be configured according to a monopole configuration, or a bipole configuration. The interface arrangement 100 may for example in alternative be configured according to an asymmetrical monopole configuration.

The interface arrangement 100 is configured to provide at least a portion of an AC waveform. Each of the multi-level converter cells WSl and WS2 is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system 120. In the context of the present application, a multi-level converter cell should be understood as a converter cell that is configured so as to be capable of providing a multiple of (two or more) voltage levels, which may be used in forming an AC voltage (waveform). The multi-level converter cell WS 1 and/or WS2 may for example comprise at least one half-bridge cell or at least one full -bridge cell, or several cascaded half-bridge cells or full-bridge cells.

Each switching element in the main thyristor bridge 130 and the interconnector 140, e.g., each pair of anti-parallel thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y, may be referred to as a converter valve, which are indicated in Figure 1 by reference numerals 131 to 136 and 141 to 143, respectively. The switching elements in each phase may be considered to be included in one converter module. Thus, the interface arrangement 100 may be considered to comprise a plurality of converter modules (e.g., one for each phase), each of which may include converter valves including switching elements in the form of pairs of anti-parallel thyristors, and each of which is electrically connected to the multi-level converter cells WS1, WS2 of the wave-shaper module 160.

Each converter module may correspond to one of the phases of the AC power system 110. The correspondence between the converter modules and the phases may be one- to-one, and so there may be a separate, or particular converter module corresponding to each phase.

By means of the converter valves 131 to 136 and 141 to 143 including switching elements in the form of pairs of anti-parallel thyristors, each converter valve 131- 136, 141-143 may be controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, thereby defining a plurality of converter states, so as to allow for or facilitate selective controlling of polarity of any voltage contribution to the AC waveform provided by the multi-level converter cells WS1, WS2. A particular converter state can hence be defined by which of the thyristors in the main thyristor bridge 130 and the interconnector 140 that are in a conducting state. However, it is to be understood that this is according to an exemplifying embodiment of the present invention, and other configurations of converter valves so as to be controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, thereby defining a plurality of converter states, are possible.

The commutation module 150 comprises three commutation cells CCR, CCY, CCB, one for each converter module, or phase. Each commutation cell CCR, CCY, CCB is electrically connected to the DC power system 120 and electrically connected to the converter valves (which according to the exemplifying embodiment of the present invention illustrated in Figure 1 are constituted by the pairs of anti-parallel thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, As will be further described in the following e.g. with reference to Figure 6, the commutation module 150 is not necessary and may be omitted.

The commutation cells CCR, CCY, CCB may be switchable so as to selectively cause e.g. at least the converter valves with which the respective commutation cell CCR, CCY, CCB is associated to enter the non-conducting state. That is, the commutation cells CCR, CCY, CCB may provide voltage for forced commutation of the converter valves (e.g., for the thyristors of the respective converter valves). One or more of the commutation cells CCR, CCY, CCB may for example comprise a full-bridge cell or several full-bridge cells, possibly cascaded.

In the context of the present application, by anti-parallel (or inverse-parallel) electrical devices such as thyristors, it is meant devices which are electrically connected in parallel but with their polarities reversed with respect to each other. Thus, in the context of the present application, by anti-parallel thyristors it is meant thyristors which are arranged in anti- parallel fashion with respect to each other. The thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y in the main thyristor bridge 130 and the interconnector 140 may 'transfer' the voltage generated by the wave-shaper module 160 to the AC side of the interface arrangement 100. The 'transferred' voltage according to an example is illustrated in Figures 2 and 3 (voltage being in arbitrary units, and the phase angle a = rot being in degrees). Note that in Figure 2, and also in Figure 4 that is described in the following, the angular frequency ω is denoted by "w". The voltages generated by the multilevel converter cells WSl, WS2 of the wave-shaper module 160 may generally conform to a sinusoidal wave shape such as indicated in Figures 2 and 3.

As also indicated in Figure 2, the voltage generated by the multi-level converter cell WSl of the wave-shaper module 160 can be described as V_m-sin(cot), where rot = a (e.g., in degrees) is the phase angle of the voltage (with ω being the angular frequency, and t being time) and V_m is the line-to-line voltage at the AC side of the interface arrangement 100 between two of the phases R, Y, B, or at the interface arrangement 100 side of the transformer 112. The voltage generated by the multi-level converter cell WS2 of the wave- shaper module 160 can be described as V_m-sin(cot+120°). The voltage generated by the multilevel converter cell WSl of the wave-shaper module 160 is shown by the dashed line in Figure 2. The voltage generated by the multi-level converter cell WS2 of the wave-shaper module 160 is shown by the dotted line in Figure 2. The sum of the voltages generated by the multi-level converter cells WSl and WS2 of the wave-shaper module 160 is shown by the solid line in Figure 2.

Figure 3 indicates which thyristors that are conducting during various phase angle intervals. The thyristors are denoted T1-T9 in Figure 3, where Tl corresponds to T_lx or Ti_y, T2 corresponds to T_2x or T_2y, and so on. As mentioned in the foregoing, the thyristors which in Figure 1 are denoted by subscript x may conduct current when the AC current is positive, and the thyristors which in Figure 1 are denoted by subscript y may conduct current when the AC current is negative.

The voltage contribution generated by the multi-level converter cell WS 1 of the wave-shaper module 160 is shown by the bold, dashed line in Figure 3. The voltage contribution generated by the multi-level converter cell WS2 of the wave-shaper module 160 module is shown by the bold, dotted line in Figure 3. The sum of the voltage contributions generated by the multi-level converter cells WSl and WS2 of the wave-shaper module 160 is shown by the bold, solid line in Figure 3. These voltages have a positive polarity. The polarity inversed voltages are indicated in Figure 3 by the solid lines which are not drawn in bold.

As indicated in Figures 2 and 3, the transition from one converter state to the other may take place when the voltage across one of the multi-level converter cells WSl,

WS2 is zero. The commutation module 150 may provide a voltage of appropriate polarity to commutate the outgoing thyristor, i.e. to switch the outgoing thyristor into a non-conducting state. The average value of the sum of the voltages generated by the multi-level converter cells WS1 and WS2 of the wave-shaper module 160 shown in Figures 2 and 3 corresponds to or equals the DC bus voltage V_s.

The relationship between the DC bus voltage Vs and the peak value of line-to- line voltage V_m (e.g., the voltage between two line conductors such as two of the phases at a given point) at the AC side of the interface arrangement 100, or at the interface arrangement 100 side of the transformer 112, can be shown to be (e.g., derived by means of HVDC converter equations, which as such are known in the art) Vs = (3/7i)V_m. According to the embodiment of the present invention illustrated in Figure 1, the subscript m in V_m denotes two of the phases R, Y, B, e.g., m = BY, RY, RB, YB, YR, BR, such as indicated in Figure 3.

Thereby, the voltage at the AC side of the interface arrangement 100 may be fixed for a given DC bus voltage. Since the AC voltage of the AC power system 110 must be changed in order to change reactive power that is consumed by the interface arrangement 100 and/or supplied by the interface arrangement 100 to the AC power system 110, the reactive power that is consumed by the interface arrangement 100 and/or supplied by the interface arrangement 100 to the AC power system 110 cannot be adjusted.

As will be described in the following, by means of embodiments of the present invention, control of reactive power exchange between the interface arrangement 100 and the AC power system 110 may be facilitated or even enabled. And further by means of embodiments of the present invention, the commutation module 150 may not be necessary for operation of the interface arrangement, and may hence be omitted (cf. the description in the following with reference to Figure 7), which may allow for simplifying the converter topology and in turn to reduce the overall cost of the interface arrangement 100 (i.e. as compared to if the commutation module 150 would be present).

In order to control reactive power exchange between the interface arrangement

100 and the AC power system 110, the interface arrangement 100 may be configured and/or arranged such that the amplitude of the voltage at the AC side of the interface arrangement 100 is controllable and/or adjustable. For example, the amplitude and shape of the voltage generated by the wave-shaper module 160 could be adjusted by way of operation of the thyristors Ti_x, Ti_y; T_2x, T_2y; ... ; T_9x, T_9y in the main thyristor bridge 130 and the

interconnector 140 such that the voltage 'transferred' to the AC side of the interface arrangement 100 has an average value which is unchanged and equals the DC bus voltage Vs.

Each of the thyristors Ti_x, Ti_y; T_2x, T_2y; ... ; T_9x, T_9y may have a controllable firing angle, which as such is known in the art. By means of the firing angle of the thyristors Ti_x, Ti_y; T_2x, T_2y; ... ; T_9x, T₉ being controllable, shape and amplitude of any voltage contribution to the AC waveform provided by the multi-level converter cells WS1, WS2 are adjustable by controlling firing angle of the thyristors Ti_x, Ti_y; T_2x, T_2y; ... ; T_9x, T₉ of the converter valves 131-136, 141-143. According to one or more embodiments of the present invention, the firing angle of the thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y may be controlled, or 'delayed', with respect to the situation illustrated in Figures 2 and 3.

As will be discussed further in the following, by means of such a delay g in the firing angle of the thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y, an additional degree of freedom in the relation between the DC bus voltage Vs and the line-to-line voltage V_m at the AC side of the interface arrangement 100, or at the interface arrangement 100 side of the transformer 112 may be attained, which may allow for or facilitate controlling reactive power exchange between the interface arrangement 100 and the AC power system 110.

The voltages generated by the multi-level converter cells WS1 and WS2 of the wave-shaper module 160 and which are 'transferred' to the AC side of the interface arrangement 100, or the interface arrangement 100 side of the transformer 112, by way of the converter valves 131-136, 141-143 while employing a delay g in the firing angle of the thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y are shown in Figures 4 and 5 (voltage being in arbitrary units, and the phase angle a = rot being in degrees). Thus, Figures 4 and 5 are similar to and correspond to Figures 2 and 3, respectively. However, while Figures 4 and 5 illustrate the situation where a delay g in the firing angle of the thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y is employed, Figures 2 and 3 illustrate the situation where no delay in the firing angle of the thyristors T_lx, T_ly; T_2x, T_2y; ... ; T_9x, T_9y is employed.

The average of the sum of voltages generated by the multi-level converter cells

WS1 and WS2 of the wave-shaper module 160 and 'transferred' to the AC side of the interface arrangement 100, or the interface arrangement 100 side of the transformer 112, may while employing such a delay g in the firing angle of the thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y be equal to the DC bus voltage V_s, and may be expressed according to the following relation (which e.g. may be derived by means of HVDC converter equations, which as such are known in the art):

V_s = - -V_m sm(wt - 120°) d(wt) = - [V_m cos(wt - 120°)Y ^g> = - V_m cos g

Ύϋ J Q Ύϋ 9 IT.

This relation may be re- written according to cos(g) = (3/π) (Vs / V_m). Based on the desired reactive power exchange, the resulting V_m may be determined or estimated. Then, the delay g of the firing angle of the thyristors may be set accordingly so as to obtain the resulting V_m, given the relation between Vs, V_m, and g (e.g., as per the relation cos(g) = (3/π)

The interface arrangement 100 comprises a control and processing module 170 configured to control operation of the interface arrangement 100. As per the discussion of the relation between Vs, V_m, and g, the control and processing module 170 may be configured to, based on a desired reactive power exchange between the interface arrangement 100 and the AC power system 110, determine a corresponding AC voltage at the AC side of the interface arrangement 100, or at the interface arrangement 100 side of the transformer 112. That is to say, the control and processing module 170 is configured to determine an AC voltage of the AC power system 110 that will correspond to a reactive power that is consumed by the interface arrangement 100 and/or supplied by the interface arrangement 100 to the AC power system 110. The control and processing module 170 may be configured to, based on the voltage of the DC power system 110 (e.g., the DC bus voltage V_s) and the determined AC voltage, determine a firing angle of the thyristor of each converter valve 131-136, 141-143 that will result in a or any voltage contribution to the AC waveform provided by the multi- level converter cells WS1, WS2 conforming to the determined AC voltage. The control and processing module 170 may be configured to control the firing angle of the thyristor of each converter valve 131-136, 141-143 in accordance with the firing angle as determined for the respective thyristor. Thus, as per the discussion in the foregoing with respect to the relation between Vs, V_m, and g, by controlling the firing angle of the thyristor of each converter valve 131-136, 141-143 in accordance with the firing angle as determined for the respective thyristor, an AC voltage at the AC side of the interface arrangement 100 may be achieved that will correspond to the desired reactive power exchange between the interface arrangement 100 and the AC power system 110. Thereby, reactive power exchange between the interface arrangement 100 and the AC power system 110 may be controlled. In turn, this may reduce or even eliminate need for any capacitor filters for reactive power compensation on the AC side of the interface arrangement 100, which may allow for or facilitate a reduction in both size and cost of the overall power transmission system.

According to one or more embodiments of the present invention, the same firing angle may be set for all thyristors Ti_x, Ti_y; T_2x, T_2y; ...; Tg_x, Tg_y in the main thyristor bridge 130 and the interconnector 140.

The control and processing module 170 may include or be constituted for example by any suitable central processing unit (CPU), microcontroller, digital signal processor (DSP), Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA), etc., or any combination thereof. The control and processing module 170 may optionally be capable of executing software instructions stored in a computer program product e.g. in the form of a memory. The memory may for example be any combination of read and write memory (RAM) and read only memory (ROM). The memory may comprise persistent storage, which for example can be a magnetic memory, an optical memory, a solid state memory or a remotely mounted memory, or any combination thereof.

As schematically illustrated in Figure 1, the control and processing module 170 may be communicatively coupled to the main thyristor bridge 130 and to the interconnector 140, and possibly to the wave-shaper module 160. The communicative coupling between the control and processing module 170 and the main thyristor bridge 130, the interconnector 140, and the wave-shaper module 160 respectively, may be wired and/or wireless and may for example be based on any appropriate communication technique or protocol as known in art for transmission of data, signals, messages, instructions, etc. The control and processing module 170 may be directly or indirectly (e.g., via a relay or the like) communicatively coupled to individual converter valves 131-136, 141-143 or individual multi-level converter cells WSl, WS2. The communicative coupling between the control and processing module 170 and the main thyristor bridge 130, the interconnector 140 and the wave-shaper module 160, respectively, may be two-way or one-way, and should at least allow for communication from the control and processing module 170 to the main thyristor bridge 130 and the interconnector 140.

It should be understood that the interface arrangement 100 may include further components than those illustrated in Figure 1, for example one or more inductors, (harmonic) filters, transformers, capacitors, etc. Such components, which are not essential to

embodiments of the present invention, are however not illustrated in Figure 1.

Figure 6 is a schematic circuit diagram of an interface arrangement 100 according to an embodiment of the present invention. The interface arrangement 100 illustrated in Figure 6 is similar to the interface arrangement 100 illustrated in Figure 1. The same reference numerals in Figures 1 and 6 denote similar or the same components, and having the same or similar function. The interface arrangement 100 illustrated in Figure 6 differs from the interface arrangement 100 illustrated in Figure 1 in that the commutation module 150 included in the interface arrangement 100 illustrated in Figure 1 is omitted in the interface arrangement 100 illustrated in Figure 6.

As described in the foregoing with reference to Figure 1, by means of the converter valves 131 to 136 and 141 to 143 including switching elements in the form of pairs of anti-parallel thyristors, each converter valve 131-136, 141-143 may be controllably switchable between conducting states with a selected current conduction direction and a nonconducting state, thereby defining a plurality of converter states, so as to allow for or facilitate selective controlling of polarity of any voltage contribution to the AC waveform provided by the multi-level converter cells WSl, WS2.

By way of the controlling of the firing angle of the thyristor of each converter valve 131-136, 141-143, voltage across the multi-level converter cells WSl, WS2 may become non-zero at change from one converter state to another converter state, which can be seen in Figure 4. This non-zero voltage across the multi-level converter cells WSl, WS2 may be employed for commutating one or more of the thyristors T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y. Thus, commutation of the thyristor of at least one converter valve 131-136, 141-143 for changing from the one converter state to the other converter state may be effected by means of the non-zero voltage across the multi-level converter cells WSl, WS2 reverse-biasing the thyristor of the at least one converter valve 131-136, 141-143. For example, a negative voltage should be applied across the outgoing thyristor for a certain time period, before it can block the forward voltage. This time period is usually referred to as extinction angle. Thus, a negative polarity of the wave-shaper module 160 (or one of the multi-level converter cells WS1, WS2 included in the wave-shaper module 160) should be maintained during a period from a time t = (g/ω) when the incoming thyristor is triggered into conduction (e.g. by a gate pulse being applied to the incoming thyristor) to a time (t + T₀ff), with T_Qff being the extinction time-duration of the thyristor. After that time duration, the polarity of the wave-shaper module 160 (or one of the multi-level converter cells WS1, WS2 included in the wave-shaper module 160) may be made positive.

For illustrating principles of embodiments of the present invention, consider as an example where, just before the commutation process, the thyristors T_7x, T_6y and T_5x are conducting current, and the currents I_R, Ιγ and ¾ are considered to be positive, negative and positive, respectively. The 'initial' converter state can hence be denoted (5, 6, 7). According to the example, the 'final' converter state is to be (6, 1, 8), with the thyristors Ti_x, T_6y and Tg_x conducting current. The transition should take place from T_5x to Tg_x in phase B, and from T_7x to Tix in phase R. The multi-level converter cell WS1 is responsible for the commutation. When a gate pulse is released to the incoming thyristor at rot = g, the polarity of the voltage of the multi-level converter cell WS1 is negative, as illustrated in Figure 7. The voltage contribution generated by the multi-level converter cell WS1 of the wave-shaper module 160 is shown in Figure 7 by the dotted line. The voltage contribution generated by the multi-level converter cell WS2 of the wave-shaper module 160 is shown in Figure 7 by the dashed line. This negative polarity of the voltage of the multi-level converter cell WS 1 facilitates the transition from thyristor T_7x to thyristor Ti_x in phase R and reverse-biases the outgoing thyristor T_7x. However, the transition from thyristor T_5x to thyristor Tg_x may not take place at this point, since thyristor T_5x is not reverse biased by the wave-shaper module 160. Assuming that T₀ff is the extinction time-duration of the thyristor, the negative polarity of the voltage of the multi-level converter cell WS 1 should preferably be maintained until t = (g/co) + T_Qff (cf . Figure 7). After this time duration, the polarity of the multi-level converter cell WS1 may be made positive, since thyristor T_7x retains its forward blocking capability. During the time period t = (g/ω) to t = (g/co)+ T_Qff, the thyristors Ti_x, T_6y and T_5x are conducting. And after t = (g/co) + T₀ff, the polarity of the voltage of the multi-level converter cell WS1 is positive, which facilitates the transition from thyristor T_5x to thyristor Tg_x. The 'final' converter state (6, 1, 8) has thereby been attained.

For the three-phase arrangement described in the foregoing with reference to Figures 1 to 7, at any given commutation instant, two of the three phases R, Y, B may participate in the commutation process. In the example described in the foregoing of commutation of thyristors using non-zero voltage across the multi-level converter cells WS1, WS2 at change from one converter state to another converter state (e.g., from (5, 6, 7) to (6, 1, 8) according to the example), thyristors in the phases R and B are involved. Further, in the above example, the commutation in phase R is aided by the multi-level converter cell's WS 1 voltage polarity, while the commutation in phase B takes place only when the voltage of the multi-level converter cell WS 1 changes polarity. However, based on the direction of current at the instant of commutation, commutation of thyristors in both of two of the phases, or in one of the phases, or in none of the phases can be aided by voltage across the wave- shaper module 160 module.

Figure 8 is a schematic flowchart of a method 400 according to an embodiment of the present invention.

The method 400 is for controlling operation of an interface arrangement (e.g., an interface arrangement 100 such a described in the foregoing with reference to Figure 1 or 6) configured to electrically couple an AC power system with a DC power system and to convert DC power to AC power, or vice versa. The interface arrangement is configured to provide at least a portion of an AC waveform. The interface arrangement comprises a plurality of multi-level converter cells, each of which is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system. The interface arrangement comprises a plurality of electrically connected converter modules. Each of the converter modules comprises at least one converter valve electrically connected to the plurality of multi-level converter cells and comprising at least one thyristor having a controllable firing angle. Shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells are adjustable by controlling firing angle of the at least one thyristor of each converter valve.

The method 400 comprises, based on a desired reactive power exchange between the interface arrangement and the AC power system, determining a corresponding AC voltage at the AC side of the interface arrangement, 410. Based on the voltage of the DC power system and the determined AC voltage it is determined a firing angle of the at least one thyristor of each converter valve that will result in a or any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells conforming to the determined AC voltage, 420. The firing angle of the at least one thyristor of each converter valve is controlled in accordance with the firing angle as determined for the respective thyristor, 430.

Referring now to Figure 9, there is shown a schematic view of computer- readable means 51, 52 carrying computer program code according to embodiments of the present invention. The computer-readable means 51, 52 or computer program code is configured to execute or run in a control and processing module according to an embodiment of the present invention, e.g. a control and processing module 170 as described above with reference to Figure 1 or 6. The computer-readable means 51, 52 or computer program code is configured to, when executed or run in the control and processing module, perform a method according to an embodiment of the present invention, e.g. as described above with reference to Figure 8. The computer-readable means 51, 52, or computer readable storage mediums, shown in Figure 9 include a Digital Versatile Disc (DVD) 51 and a floppy disk 52. Although only two different types of computer-readable means 51, 52 are depicted in Figure 9, the present invention encompasses embodiments employing any other suitable type of computer- readable means or computer-readable digital storage medium, such as, but not limited to, a nonvolatile memory, a hard disk drive, a CD, a Flash memory, magnetic tape, a USB memory device, a Zip drive, etc.

In conclusion there are disclosed a control and processing module and a method of controlling operation of a thyristor-based interface arrangement configured to electrically couple an AC power system with a DC power system, and to convert DC power to AC power, or vice versa. The interface arrangement comprises a plurality of converter valves, each comprising at least one thyristor. By means of controlling the firing angle of the at least one thyristor of each converter valve, reactive power exchange between the interface arrangement and the AC power system may be controlled. A system comprising the interface arrangement and the control and processing module is also disclosed.

While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A control and processing module (170) configured to control operation of an interface arrangement (100) which is configured to electrically couple an alternating current, AC, power system (110) with a direct current, DC, power system (120), and to convert DC power to AC power, or vice versa, the interface arrangement being configured to provide at least a portion of an AC waveform, wherein the interface arrangement comprises a plurality of multi-level converter cells (WS1, WS2), each of which is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system, and a plurality of electrically connected converter modules each of which comprises at least one converter valve (131-136, 141-143) electrically connected to the plurality of multi-level converter cells and comprising at least one thyristor (T_lx, T_ly; T_2x, T_2y; ... ; Tg_x, Tg_y) having a controllable firing angle, wherein shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells are adjustable by controlling firing angle of the at least one thyristor of each converter valve, and wherein the control and processing module is configured to:

based on the voltage of the DC power system and the determined AC voltage, determine a firing angle of the at least one thyristor of each converter valve that will result in a voltage contribution to the AC waveform provided by the plurality of multi-level converter cells conforming to the determined AC voltage; and

control the firing angle of the at least one thyristor of each converter valve in accordance with the firing angle as determined for the respective thyristor.

2. A control and processing module according to claim 1, wherein the control and processing module is further configured to control the firing angle of each thyristor individually, or to control the firing angles of all thyristors as a group.

3. A control and processing module according to claim 1, wherein the control and processing module is further configured to control the firing angle of the at least one thyristor of each converter valve within a range between about 5° to about 27°.

4. A system (100, 170) comprising: an interface arrangement (100) configured to electrically couple an alternating current, AC, power system (110) with a direct current, DC, power system (120), and to convert DC power to AC power, or vice versa, the interface arrangement being configured to provide at least a portion of an AC waveform, wherein the interface arrangement comprises a plurality of multi-level converter cells (WS 1, WS2), each of which is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system, and a plurality of electrically connected converter modules each of which comprises and at least one converter valve (131- 136, 141- 143) electrically connected to the plurality of multi-level converter cells and comprising at least one thyristor (T_lx, T_ly; T_2x, T_2y, . .. , Tg_x, Tg_y) having a controllable firing angle, wherein shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells are adjustable by controlling firing angle of the at least one thyristor of each converter valve; and

a control and processing module (170) according to any one of claims 1-3 configured to control operation of the interface arrangement.

5. A system according to claim 4, wherein at least one of the plurality of converter valves is line-commutated.

6. A system according to claim 4 or 5, wherein at least one of the plurality of converter valves comprises at least one anti-parallel thyristor pair.

7. A system according to any one of claims 4-6, wherein the interface

arrangement is configured such that each converter valve is controllably switchable between conducting states with a selected current conduction direction and a non-conducting state, thereby defining a plurality of converter states, so as to selectively control polarity of any voltage contribution to the AC waveform provided by the multi-level converter cells; and wherein by way of the controlling of the firing angle of the at least one thyristor of each converter valve voltage across at least one of the multi-level converter cells is non-zero at change from one converter state to another converter state, whereby

commutation of the at least one thyristor of at least one converter valve for changing from the one converter state to the other converter state can be effected by means of the non-zero voltage across at least one of the multi-level converter cells reverse-biasing the at least one thyristor of the at least one converter valve.

8. A system according to any one of claims 4-7 wherein the AC power system comprises a plurality of phases, and wherein each of the converter modules corresponds to one of the phases.

9. A system according to any one of claims 4-8, wherein the at least one multilevel converter cell comprises a half-bridge cell or a full-bridge cell.

10. A system according to any one of claims 4-9, wherein the interface

arrangement comprises an HVDC converter.

11. A power system (110, 100, 120) comprising:

an alternating current, AC, power system (110);

a direct current, DC, power system (120); and

a system (100, 170) according to any one of claims 4-10, wherein the interface arrangement (100) of the system is configured to electrically couple the AC power system with the DC power system.

12. A method (400) of controlling operation of an interface arrangement (100) which is configured to electrically couple an alternating current, AC, power system (110) with a direct current, DC, power system (120), and to convert DC power to AC power, or vice versa, the interface arrangement being configured to provide at least a portion of an AC waveform, wherein the interface arrangement comprises a plurality of multi-level converter cells (WS1, WS2), each of which is configured to provide a voltage contribution to the AC waveform based on voltage of the DC power system, and a plurality of electrically connected converter modules each of which comprises at least one converter valve (131-136, 141- 143) electrically connected to the plurality of multi-level converter cells and comprising at least one thyristor (T_lx, T_ly; T_2x, T_2y; . .. ; Tg_x, Tg_y) having a controllable firing angle, wherein shape and amplitude of any voltage contribution to the AC waveform provided by the plurality of multi-level converter cells are adjustable by controlling firing angle of the at least one thyristor of each converter valve, the method comprising:

based on a desired reactive power exchange between the interface arrangement and the AC power system, determining (410) a corresponding AC voltage at the AC side of the interface arrangement;

based on the voltage of the DC power system and the determined AC voltage, determining (420) a firing angle of the at least one thyristor of each converter valve that will result in a voltage contribution to the AC waveform provided by the plurality of multi-level converter cells conforming to the determined AC voltage; and

controlling (430) the firing angle of the at least one thyristor of each converter valve in accordance with the firing angle as determined for the respective thyristor.

13. A computer program product configured to, when executed in a control and processing module (170) according to any one of claims 1-3, perform a method (400) according to claim 12.

14. A computer-readable storage medium (51; 52) on which there is stored a computer program product configured to, when executed in a control and processing module (170) according to any one of claims 1-3, perform a method (400) according to claim 12.